People measure voltages for many reasons. Typical electronic test equipment measures a voltage at a point within some environment of interest by directly contacting the point with a probe. Examples of such equipment include AC and DC voltmeters, oscilloscopes and many types of RF test equipment, especially if the concept of “probing” includes propagating electromagnetic energy inducing a voltage in a suitable antenna. Such measurements are the stock-in-trade of those engaged in designing, developing and testing electrical devices, including diagnosing and repairing such devices.
In many voltage measurement applications, it is practical to make a voltage measurement through physical contact of a probe with a conductor. However, there are cases where contactless measurements are highly desirable, if not absolutely necessary. Typically, a contactless measurement is needed when the conductor whose voltage is to be measured does not have an exposed portion that can be contacted. For example, conductors that are on the inner layers of a multi-layer printed circuit board, e.g., transmission lines, do not have such exposed portion. Ball grid arrays are another example.
Another example is the conductors of a flat-panel display (FPD) that, for example, constitutes part of a computer monitor or a large-format, high-definition television and that has been fabricated to a point at which the conductors have been encapsulated and can no longer be contacted with a probe. Even if they are not yet encapsulated, the conductors are microscopic and fragile. Even when the test is performed at a point in the manufacturing process before the conductors are enclosed by a protective covering, the number of locations to be probed is large, and each location is delicate and can easily be damaged by contact with a probe. To obtain a reasonable test throughput, possibly thousands of measurement locations have to be probed simultaneously. Each location is distanced from its neighboring locations by the pitch of the display, which is typically a few hundred micrometers. A suitable multi-probe probe head is typically expensive and its use incurs the risk that a probe will damage the flat-panel display under test, resulting in a failure either instantly or some future time. The cost of such a probe head and the cost of damaged units contribute significantly to the cost of flat-panel displays.
Non-contact voltage measurement techniques that discover the presence of a voltage on a conductor include those based on the Pockels effect and involve measuring the effect of an electric field on the polarization of light passing through an optically-transmissive medium located in the electric field. While this technique has its advantages, it fails to provide an adequate sensitivity and spatial resolution. Another non-contact voltage measurement technique involves scanning the device under test with an electron beam to detect the emission of secondary electrons. This test technique has a better voltage sensitivity and spatial resolution, but it is performed in a high vacuum, which significantly increases the test time.
What is needed, therefore, is a voltage measurement technique that is capable of contactless voltage measurement and that has a voltage sensitivity, spatial resolution and operational convenience suitable for rapidly measuring voltage at multiple, small measurement locations in a device under test. What is also needed is a voltage measurement technique that can be used to make simultaneous contactless voltage measurements in a relatively large area of a device under test.